Liquid crystal display devices display images by using a phenomenon in which the refractive index of liquid crystals is changed in accordance with a change in alignment of liquid crystal molecules when an electric field is applied to the liquid crystals, that is, an electro-optic effect of liquid crystals. In addition, the change in alignment of liquid crystal molecules follows a change in the voltage of an electric signal (a video signal) based on image information.
A response time, from when an applied voltage is changed to when a change in alignment of liquid crystal molecules converges, of liquid crystals used for a liquid crystal display device is approximately ten and several milliseconds in general, whereas, for example, one frame period is approximately 17 msec when a liquid crystal display device is driven at a frame frequency of 60 Hz. Thus, since a percentage of the response time of liquid crystals in one frame period is high, a change in the transmittance of a liquid crystal element is likely to appear a blur of a moving image. In order to improve image quality of an moving image, a response time can be shortened to some level by employing overdrive that changes alignment of liquid crystals quickly by setting a voltage applied to a liquid crystal element to be high level temporally, or by devising a countermeasure such as improving liquid crystals themselves. However, even if the response time is shortened, it takes approximately several milliseconds and image quality of a moving image still has a lot to be improved.
There is another reason why a moving image of a liquid crystal display device appears blurred, in addition to the above-described response time of liquid crystals, that is, the liquid crystal display device employs hold-driving in which a voltage is always applied to a liquid crystal element. Since human eyes have a property of recognizing afterimages, when any gray levels except black is consequently displayed, the human eyes cannot follow changes in the gray levels with hold-driving, whereby a moving image is likely to be seen as a blur.
Then, in order to solve blurs caused by both the response time of liquid crystals and hold-driving, impulsive driving has been proposed in which a backlight is turned off to display black during a period when a change in alignment of liquid crystal molecules is considerable. By employing impulsive driving, a backlight can be turned off during a period when a change in the transmittance of a liquid crystal element is considerable and afterimages can be prevented from being left in human eyes, whereby a blur of a moving image can be solved.
Reference 1 (Japanese Published Patent Application No. H11-202286) discloses a driving method in which traces in displaying moving images are eliminated by turning on a light when liquid crystals response after data is written to a pixel.